The Utah Transportation Center, in conjunction with CAIT at Rutgers, sponsored a study that investigated the distribution factors and load ratings of a continuous, steel I-girder bridge. The SH-52 Bridge over the Snake River is located on the Idaho-Oregon border near Payette, Idaho. The bridge was built in the 1950’s and presently supports two lanes of traffic.

A finite-element model of the bridge was calibrated with the results from a live-load test. For the live-load test, the bridge was instrumented at nine longitudinal cross section locations with 62 strain gauges attached on the girders, stringers, and intermediate diaphragms. The live-load was applied with two heavy trucks that were driven along three predetermined load paths.

The calibrated finite-element model was used to quantify moment distribution factors and load ratings for the bridge. The finite-element distribution factors were compared to those calculated according to the AASHTO Standard and AASHTO LRFD Specifications. The distribution factors from both AASHTO codes were found to be unconservative for the girders and overly conservative for the stringers.

The model was also used to quantify the effect of the transverse diaphragm members on the live-load distribution. Distribution factors were calculated with and without the diaphragm members. The diaphragms were found to increase the distribution of moments by over 20% for both positive and negative moments.